The present invention relates to a developer for positive photoresists, having improved properties with respect to the resolution and edge sharpness and definition, obtainable in the developed resist image, and with respect to its developing activity and capacity, and to stability.
Photoresist materials of both the positive and negative types are used in the photolithographic transfer of image structures in diverse ways.
In semiconductor technology and microelectronics, positive photoresist materials are of particular importance since, in the transfer of extremely fine image structures to conductor and semiconductor substrates, they guarantee a resolution which nowadays is still the best possible.
To produce positive resist images on a substrate, the latter is coated with the resist material in the form of a lacquer, and an image pattern is then transferred by exposure, for example through a mask, to the photoresist layer. In a subsequent developing process, the exposed photoresist areas are dissolved away, whereby a positive relief image is then formed on the substrate. Further structuring or modification, for example by etching, by metal coating or doping can then be carried out in the bared substrate areas.
The positive photoresist materials used are usually light-sensitive compositions based on phenol/formaldehyde condensation products of the novolak type as the resin components and compounds of the o-diazoquinone type as the light-sensitive components. By the action of light, usually in the ultraviolet range, the solubility of the exposed photoresist areas in an aqueous-alkaline medium is drastically increased. Such positive photoresist materials can therefore be developed in aqueous-alkaline developer systems. The progress in miniaturization in semiconductor technology, in paticular the VLSI technique (Very Large Scale Integration) for the production of highly and very highly integrated electronic components and circuits requires the highest possible degree of accuracy of reproduction in these photolithographic processes. Development as a process step is here of particular importance.
The quality of the positive photoresist relief structures to be obtained, with respect to resolution, edge definition, remaining layer thickness and surface quality is decided in particular by the influence of the developer during development, in addition to the precision of the exposure apparatus and the properties of the photoresist.
Positive photoresist developers of the state of the art customarily are aqueous solutions of compounds producing an alkaline pH value. In most case, they contain alkalis, such as alkali metal hydroxides, or alkali metal salts, for example alkali metal silicates or phosphates. For stabilizing the pH value of the solution, the developers can be buffered and, for improving wettability, they can contain a variety of surfactants.
The known developers have, however, the disadvantage that they also attack unexposed image areas of the resist layer to a not inconsiderable degree and cause noticeable layer removal. This manifests itself, in particular, also in the boundary regions between the exposed and unexposed image areas where, due to the layer removal by the developer, the 90.degree. edges, desirable in the ideal case, of the remaining resist relief structures are bevelled and rounded to a considerable extent. This leads to only moderate contrast, a marked deterioration of the possible resolution, as given by the optical parameters, and an inaccurate image reproduction for the subsequent processes, such as etching, coating or doping of the substrate.
Moreover, known developers have only a limited stability and developing capacity. During storage and use, they show changes in the composition, so that reproducibility of the development results is not ensured over a prolonged period. It is therefore usual continuously to monitor the development processes on the substrate, which is to be developed, by means of methods which are expensive in apparatus, and to detect the end point of development. Furthermore, flocculation and deposition of impurities and degradation products of the developer and resist material are observed in the developer solution and, a particularly adverse factor, on the substrate.
There is therefore a demand for improved positive photoresist developers for microelectronics, in particular the VLSI technique.